1. Field of the Invention
This application relates to a rotary positive displacement (PD) pump, such as a twin screw pump, an internal or external gear pump, a lobe pump, a vane pump or a progressive cavity pump; and more particularly, relates to techniques for tuning such a rotary PD pump in order to determine pump flow, including a twin screw PD pump.
2. Brief Description of Related Art
Many different type or kinds of pumps, including rotary positive displacement pumps, are known in the art. By way of example, some known pumps and shortcomings associated with the same are set forth below:
U.S. Pat. No. 6,591,697, entitled “Method for Determining Pump Flow Rates Using Motor Torque Measurements,” which is hereby incorporated by reference in its entirety, discloses a methodology that is based on the relationship of torque and speed versus pump flow rate and the ability to regulate pump flow using a Variable Frequency Drive (VFD) to adjust centrifugal pump speed. The technique used by Mr. Henyan in the '697 patent relies on calibrating pump flow at several speeds and determining a flow value based on calibrated torque vs flow curves at several speeds. An interpolation method is used to determine flow between calibrated curves. Data for the calibrated flow curves are taken at zero flow (closed valve condition) at several speeds. A positive displacement pump cannot be operated at closed valve condition without pressure relief valves or bypass piping as the pump will continue to increase pressure and power until either a shaft or gear breaks or rupture occurs either in the system piping or pump casing. Another shortcoming is that the '697 patent relies on taking calibrated data at the factory which makes the variable frequency drive specific to the pump tested. Also, the invention has no provision for adjusting flow accuracy as pump wear occurs. Mr. Henyan's invention relates to centrifugal pumps where torque is proportional to the square of the speed change. In a rotary positive displacement pump torque is constant regardless of speed. Therefore mathematical relationships and the equations governing flow between centrifugal and rotary positive displacement pumps are completely different. Therefore, Mr. Henyan's technique is applicable to centrifugal pumps only and cannot be applied to positive displacement pumps.
U.S. Pat. No. 7,945,411 B2, entitled “Method for Determining Pump Flow Without the Use of Traditional Sensors” which is hereby incorporated by reference in its entirety, discloses a technique that samples speed and power data at closed valve condition to correct the published pump curve for actual performance. Normalized power curves along with speed and power data taken from a Variable Frequency Drive (VFD) are used to calculate flow. The technique used in the '411 patent by Mr. Kernan utilizes speed and power data at closed valve condition to adjust published pump performance for actual performance. A positive displacement pump cannot be operated at closed valve condition without a pressure relief valve or bypass piping as it will continue to increase pressure and power until either a shaft or gear breaks or rupture occurs either in the system piping or pump casing. Pump flow is calculated by a polynomial power equation and normalized power curves. In a centrifugal pump pressure varies as the square of the speed change and power varies as the cube of the speed change. A centrifugal pump is not a positive displacement machine and as such the capacity output will vary based on the resistance at the pump outlet. Less resistance will give more flow; more resistance less flow. A rotary positive displacement pump is a positive displacement machine where a defined volume of flow is positively displaced for each revolution of the pump shaft regardless of pressure at the outlet (unless blocked). For a rotary positive displacement pump flow is proportional to a speed change regardless of outlet pressure. There is slip which occurs which reduces the theoretical displacement due to clearances, pressure, viscosity and speed. Power typically is proportional to a speed change in rotary positive displacement pumps at constant pressure; in a centrifugal pump power varies as the cube of the speed change. Mathematical relationships and the equations governing flow between centrifugal and rotary positive displacement pumps are completely different. Therefore, Mr. Kernan's technique is applicable to centrifugal pumps only and cannot be applied to positive displacement pumps.
In the prior art, it is known to use calculations from resource material, e.g., a pump handbook for a positive displacement pump. However, one disadvantage with this approach is that calculation techniques such as those presented in the pump handbook require knowledge of difficult to determine variables such as pump geometry factor and slip coefficient. These calculation techniques cannot compensate for pump performance which deviates from published performance calculations.
In the prior art, it is known to use external flow meters; however, external flow meters can add cost and complexity to the overall drive system.
None of the aforementioned techniques described herein may be used for determining pump flow in rotary positive displacement pumps, as set forth below and herein.